The Department of Agriculture, Forestry and Fisheries (DAFF) has recently granted permission to release the pereskia stem-wilter (Catorhintha schaffneri) in South Africa as a biological control agent to pereskia.
Pereskia grows in ecologically sensitive and difficult to access areas making control using herbicides and physical removal difficult. The plant can also reproduce from small fragments, so after clearing operations the tiny fragments of the plant that are left behind will grow into new plants. The weed grows intertwined with indigenous plants so pereskia cannot be removed without killing indigenous plants too.
The biological control agent (pereskia stem-wilter) feeds on the shoot tips of pereskia and is very damaging to the plants making it a perfect biological control agent.
Pereskia (Pereskia aculeata) is an invasive alien plant that was brought to South Africa in 1858 as a botanical curiosity. It destroys South African native biodiversity by smothering indigenous plants and killing large forest trees which collapse under the weight of the weed. Infested forests become degraded ecosystems with very low levels of biodiversity.
Rhodes University lead researcher, Dr Iain Paterson from the Department of Zoology and Entomology said they have moved pereskia stem-wilter out of quarantine and mass rearing efforts are underway at the Waainek greenhouses. The pereskia stem-wilters are fed plant cuttings from infestations growing around Grahamstown.
“It is one of the worst weeds in South Africa” said Dr Paterson. The Working for Water Programme (WfW) of the Department of Environmental Affairs: Natural Resource Management programme (DEA:NRM) funded research into biological control of pereskia because of the damage it is causing to indigenous forests.
Dr Paterson has been working with the insect since 2007 and has been raising them in quarantine since 2012. The pereskia stem-wilter was imported into quarantine and was subjected to host specificity testing. This testing is designed to identify which plants the insect can feed on.
“After many years of research it is now clear that the pereskia stem-wilter can only survive on pereskia and will die if it is forced to feed on any other plant. If the insect is given a choice it will always choose to feed on pereskia. This means that if the pereskia stem-wilter is released in South Africa it will feed on pereskia only and will not harm any indigenous plants or crops,” he explained.
Mr Vuyani Ntyinkala has now been tasked with mass rearing the pereskia stem-wilter at the Waainek greenhouses. Ntyinkala has recently joined the team having come through a training programme offered by the Department of Entomology in collaboration with Grahamstown Area Distress Relief Association (Gadra).
Mr Ntyinkala has a dislocated left hip from a car accident and despite being physically disabled he has now joined the Biological Control Research Group and is excited about his new responsibility.
“I’m looking after these insects now and we are trying to get more and more. I am hoping that there are going to be more during the summer season. As it is winter now you don’t see them, they are hiding themselves,” he explained.
His task is to keep the insects living optimally to encourage breeding, it has only been four months since his training and a few weeks since being given his new responsibility but he is thrilled with his new ability.
The pereskia is changed daily in order to ensure that the pereskia stem-wilter has plenty to eat. He also sprays the cage with water so that they have access to drinking water. Mr Ntyinkala is monitoring the pereskia stem-wilter closely and has observed them mating up at the greenhouses which is a positive sign.
“They feed on the shoot tips so this is their optimum kind of food. And then they will lay their eggs on the back of leaves but we haven’t got eggs yet because they have just moved up here, it usually takes 2-3 weeks before they lay eggs. They are mating so it usually takes 2-3 weeks after that. But because it’s cold it might be a bit slower,” added Dr Paterson.
Quarantine is a lot warmer than in the tunnels but they have done climatic matching to ensure that the pereskia stem-wilter can survive in South African temperatures. The insect which is native to Brazil will prefer the warmer conditions of South Africa’s North-East coastline which is ideally suited as pereskia is causing the most damage in those regions.
“We hope to release about 300 at each site that we identify. And the sites are all the way from Port St Johns to about Kosi Bay. There are sites in Grahamstown but it’s much warmer in KwaZulu-Natal so we will release there first,” explained Dr Paterson.
“It’s a really exciting project and I hope to learn more, because I used to use herbicides only and it does not only kill the plant you want to kill but I have learnt that these insects are specific. If it is eating this plant then it is only eating this one, it can’t damage that one,” said Mr Ntyinkala.
Mr Ntyinkala has become a firm activist for biological control agents, “If you would like to keep this plant but destroy another one, the insects we are using don’t damage the other one. They are very specific. They know which plants they like.”
Many insects can only feed on a single plant species because they have co-evolved with the plant and have therefore specialised for feeding on that plant species alone. These specialised insects may be suitable for biological control. If all the pereskia in an area is destroyed by the insect it will either disperse to another site where pereskia is present or the insect population will die out.
Mr Nytinkala is not worried about mass rearing the insects and very optimistic about his task, “I think I can get lots of these insects, they are going to grow quickly.”
Unlike herbicidal and physical removals, biological control is environmentally friendly and sustainable. It is a long term solution and only damages the target weed, not the surrounding vegetation. Mass rearing efforts will also take place at the South African Sugar Research Institute (SASRI) and releases will be made at sites infested with pereskia throughout the country. The pereskia stem-wilter is set to be released in September 2014.
“We send it to conservationists, land owners and they get it for free but then what I ask in return is that three months after release they do some very basic measurements and then I give them a data sheet and they email it to me and then I use that information to decide whether I should send them more insects,” added Dr Paterson.
Photo: Mr Vuyani Ntyinkala and Dr Iain Paterson in the greenhouse where the pereskia stem-wilter is being mass-reared.